Neutron thermometer



May 4, 1954 R. J. MOON NEUTRQN THERMOMETER Filed Aug. l5, 1946 -mFmPatented May 4, 1954 NEnrnoN THERMoMErER Robert J. Moon, Chicago, Ill.,assignor to the United States of America as represented by the UnitedStates Atomic Energy Commission Application August 15, 1946, Serial No.690,801

(Cl. Z50-$3) 7 Claims.

The present invention relates to a neutron responsive device, andparticularly to a neutron thermometer adapted for use in connection withthe operation and control of neutronic reactor systems.

A neutronic chain reactor, as the terrn is used herein, constitutes acomplete system designed for initiating and carrying out controllednuclear chain reactions The power derived from a reactor is, in general,related proportionally to the neutron flux density which is derived fromuclear ssion.

The neutron thermometer depends for its operation on the nucleartransmutation of energy by materials of high neutron capture crosssection, for example, the transmutation of boron to lithium with analpha emission of approximately 2 1n. e. v., where the energy of thealpha emission remains almost entirely within the said bombarded mediumwhich may encase hermocouplc elements. In this manner, the neutronthermometer may utilize the dissipation of the energy of the alphaemission as heat, and transform said energy into thermoelectric power byof a device of thermopile design.

As a principal object of the invention, it is intended to provide aneutron ilux indicating device, simple and compact in structure, whichis capable of giving and reproducing accurate and continuous indicationsof neutron flux density within or adjacent to a neutrcnic reactor.

It is a further object of the invention. to provide a neutron densityindicating device having suicient range and sensitivity for themeasurenient 01"' high intensity neutron ux densities as well as lowintensity neutron flux densities.

Another object of the invention is to design a neutron thermometerwherein the time for response of the thermometer to changes of neutroniiux density are of minimum order, and substantially independent of theneutron iiux density.

A further object of the invention is to design a neutron thermometerwhich is simple and compact in construction, so as to be portable andmeasuring means whereby the response is amplined by incorporating thethermopile construction of thermocouples. The neutron thermometerincludes a structure of a plurality of hot and cold junctions arrangedin such proximity so as to compensate for the ambient temperaturedeviations which produce spurious voltages, since said voltages aredeleterious in the attainment of accurate neutron iiux measurements. Themeans for producing heat at the hot junctions of the thermopile is acoating of a material of high neutron capture cross section encasing thehot junction of the thermocouples. In accordance with the presentinvention, high thermal conductivity between the neutron capture coatingand the thermocouple elements is provided so as to realize the maximumheat transfer, whereas in the prior art such neutron capture coatingshave been incorporated With heat insulating binders so as to reduce theefciency oi thermal conduction.

As will hereinafter appear, these objects are accomplished by theprovision of a thermocouple or a thermo-pile device that is temperatureresponsive to neutron irradiation to produce thermo-electromotive forcesproportional to the density of radiation. The various novel structuraland constructional arrangements contributing to the successful operationoi the apparatus of the invention and a more complete exposition of itsprinciples and mannner of operation will he found in the followingdescription and the accompanying drawings of certain preferredembodiments thereof. ln the drawings:

Fig. 1 is a more or less diagrammatic, longitudinal sectional View of aneutron thermometer embodying the present invention;

Fig. 2 is an enlarged sectional view taken on the line 2 2 of Fig. 1;

Fig. 3 is an enlarged sectional view taken on the line 3-3 oi Fig. l;and

Fig. i is a longitudinal sectional view of a thermopile segment showingthe relative positions of the cold junctions and the encased hotjunctions in the thermopile assembly.

The relative disposition of the parts of the device of a coated hotjunction type is best illustrated in Fig. l. Since the thermo-electricpower produced by a single thermocouple is relatively of smallmagnitude, it is desirable, in many instances, to employ a thermopileassembly it, rather than a single thermocouple, said assembly ii beingillustrated in the gures.

This device includes a case l2 preferably in the form of a hollowmetallic tube having at one end a metallic plug insert I6, which isundercut and inserted into said tube and secured thereto as at I4. Theopposite end of the hollow tube is closed and capped by means of ametallic cap I8, which is connected to the tube or case I2 by means of ametallic insert 20. In the present invention, the metallic case I2functions to insure equipotential thermall surface so as to reduce theeffect of yexternal thermal gradients. Spurious E. M. F.s aresubstantially eliminated by the use of the metallic case I2, when thethermopile assembly II is moved from regions of high temperature toregions of relatively low temperatures, as encountered in the neutronicreactor system and the ambient surroundings.

The case I2 is preferably constructed of a material, such as relativelypure aluminum, which has the characteristic of relatively high heatconductivity and also has a relatively low neutron capturecross-section, thus affording a neutron permeable case around thethermopile structure hereinafter described in detail.

lThe thermopile assembly I@ includes two insulating discs 22 whichconfront each other within the hollow tube I4. One disc 22 isreeiliently attached to the plug insert I6 by means of a screw 24 whichis inserted through a centrally disposed hole 28 in said disc 22 and isthreaded into the insert plug IE, a spring 25 being adjustablycompressed between the head of the screw 2li and the plug I6. Theconfronting disc 22 is rigidly attached to the insert 2Q by a screw 25.

The thermopile assembly II! comprises a plurality of thermocoupleselectrically connected in series and supported by said discs 22. Each ofthe individual thermocouple elements making up the thermopile assemblyIi! is comprised cf two lengths II and I3 of relatively iine wire ofdissimilar composition butt welded together into a continuous strand soas to provide a series oi alternately arranged hot junctions I5 and coldjunctions I1 extending between the confronting discs 22. In theparticular embodiment illustrated, the outer dimension of the tube Iiihas a diameter of about .475 inch, an inside diameter of .369 inch andlength of about 61/2 inches. The thermocouple wires II and I3 have adiameter of approximately 0.013 inch and a length of about A70 inch. Thewires II and I3 are butt welded in a nonoxidizing atmosphere, therebyforming a continuous strand of wire. The wires II are preferablycomposed of a nickel-chromium alloy, i. e., Chromel-P, and the wires I3are preferably composed of a dissimiliar material such as anickel-silicon-manganese alloy, i. e., Alumel. Other combinations ofdissimilar elements I I and I3 which have relatively high thermoelectricpower can be used in a similar manner.

The hot junctions I5 of the thermopile assembly if! are preferablycoated with boron produced by the thermal decomposition of diborane toproduce a boron coating 2I on the hot junction I5.

The continuous wire formed of the alternately arranged wires II .and I3is threaded or interlaced through openings 36 in the discs 22 andextends back and forth therebetween to form a plurality of substantiallyparallel segments i9 so arranged that the hot junctions I5 and the coldjunctions I'I of adjacent segments are symmetrically disposed injuxtaposition, whereby each hot junction of each segment is disposed inan isothermal plane at right angles to the thermopile major axis andpassing through the corresponding cold junctions of the adjacentsegments, as best seen inFig. 3.

This arrangement of the hot junctions I5 and the cold junctions I'Itends to eliminate the spurious E. M. F.s produced by the unsymmetricalnow of heat from the thermopile assembly, as in devices known in theprior art.

The two terminal end leads 32 of the thermopile assembly Iii extendthrough tubular quartz insulators .34' extending: through an associatedinsulating disc 23 and the metallic insert 20, said terminal end leads32 being connected permanently to binding posts 35. The end cap ii! isthreaded so that a probing metallic tube It@ can be easily attached tothe neutron thermometer vfor the purpose ofA scanning the internalstructure of the vneutronic reactor. The probing inetallic tube 49 is ofsmall diameter and long length,fwhereby the lead-in wires d2 ofinsulated copper or aluminum wire are threaded through the Aprobingmetallic tube 4! so as to connect electrically the thermopile terminalend leads 32 and the binding posts 36 to an'external voltage indicatingdevice such as a recording potentiometer or galvanometer.

rThe thcrmopile construction described in the foregoing constitutes anefficient neutron flux density indicating means. Due to the rather `argenumber of thermocouples used, sufficient thermo-electric power Visproduced even at a low neutron density to give reliably reproducibleindications. Further, the response characteristic ofthe thermocoupleconstruction disclosed approximates linearity; and due to the very lowheat capacity of the thermopile elements and supports, the response ispractically instantaneous throughout the operative range of the device.In addition, the low thermal capacity of the thermopile assembly It),`vin combination with the insulating discs 22, which suspends both hotjunctions 25 and cold junctions il, in air, serves to greatly minimizethe time lag in restoring the device to Y operative condition followingrapid changes in neutron -fiux density. In fact, operation of suchdevices has shown that under the most severe conditions `likely to beencountered, the time lag in order to restore the device to its normaloperating level will be in the order of a fraction of a second. Extendeduse of the device indicates that there is substantially no deteriorationof the materials of which it is constructedor changesin calibration overa considerable period of time.

It may benoted that yspurious voltages induced by varying magnetic eldsare substantially eliminated in the novel device which embodiesnon-inductive .windings because said windings possess negligibleself-inductance and are substantially resistive. According to thepresent invention, each segment is interlaced in such a manner so thatthe induced E. M. Fis are substantially neutralized, thereby eliminatingcoupling within the circuit.

The above-described neutron thermometer, as a device for monitoring theneutronic reactor at a nxed position, is sufficiently compact andportable so that the device can be moved through the reactor to measurethe distribution of neutron ux densityduring operation of the reactor.

In the foregoingr have described how thermocouple Vdevices of certainparticular constructional arrangements will be used to indicate neutronflux density. It will be apparent that various other structuralarrangements, not necessarily embodying the particular details of theembodiments disclosed, may be constructed and, accordingly, it is myintention that the accompanying claims shall be accorded the broadestpossible construction, consistent with the language appearing thereinand in the prior art.

What is claimed is:

1. A neutron thermometer comprising a thermopile in the form of acontinuous strand containing a plurality or" alternately arranged hotand cold junctions, said strand being wound into a plurality ofsubstantially parallel segments so arranged that the hot junctions ofeach segment is disposed closely adjacent to the corresponding coldjunctions of the adjacent segment, the hot junctions of said thermopilebeing coated with a material having a high neutron capture crosssection.

2. A device of the class described comprising a continuous strandnon-inductively Wound to form substantially parallel segments, saidstrand being formed of a plurality of alternately arranged thermocoupleelements of dissimilar elements joined end to end and capable ofdeveloping thermo-electric power at the junctions therebetween, the 1notjunctions of said thermocouple being coated with a material having ahigh neutron capture cross section.

3. A device of the class described comprising a continuous strandnon-inductively wound to form a plurality of substantially parallelsegments, said strand being formed of a plurality of alternatelyarranged thermocouple elements of dissimilar materials joined end to endin alternate hot and cold junctions, the hot junctions being encasedwith neutron absorbent material, each hot junction of each segment beingdisposed in a plane perpendicular to the longitudinal axis of the devicepassing through the corresponding cold junctions of the adjacentsegments.

l. A neutron indicating means comprising a plurality of thermocoupleelements of dissimilar materials capable of developing thermo-electricpower at the junctions thereof, said elements being joined end to endthereby forming alternate hot and cold junctions, the hot junctions ofsaid thermopile being coated with a material having a high neutroncapture cross section, said structure arranged between a pair of spacedinsulating discs operatively arranged so that the cold junction or eachsegment and the corresponding hot junction of adjacent segments is inthe same isothermal plane at right angles to the thermopile major axis,the continuous wire formed of said thermocouple elements beinginterlaced through said insulating discs a plurality of times and theunconnected ends of said continuous wire extending through one of saiddiscs, whereby associated lead-in connectors may be connected to saidends at one extremity of the device.

5. A device of the class described comprising a pair of insulatingmembers and a continuous wire extending back and forth between saidmembers a plurality of times to afford a plurality of substantiallyparallel segments extending in substantially straight lines between saidmembers, said Wire being formed of a plurality of alternate elements ofdissimliar materials joined together thereby forming alternate hot andcold junctions, said hot junctions being encased with materials of highneutron capture cross section.

6. A device of the class described comprising a pair of insulatingmembers and a continuous wire extending back and forth between saidmembers a plurality of times to afford a plurality of substantiallyparallel segments extending in substantially straight lines between saidmembers, said wire being formed of a plurality of alternate elements ofdissimilar materials joined together thereby forming alternate hot andcold junctions, said hot junctions being encased in boron.

7. A device of the class described comprising a pair of discsconstructed of electrically insulating material and a continuous wireextending back and forth between said discs a plurality of times toafford a plurality of substantially straight lines between said discs,said Wire being formed of alternate nickel-chromium alloy segments andnickel-silicon-manganese alloy segments joined end to end therebyforming alternate hot and cold junctions, said hot junctions beingencased with material of high neutron capture cross section.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 1,081,365 Coblentz Dec. 16, 1913 2,579,994 Zinn Dec. 25, 1951FOREIGN PATENTS Number Country Date 213,868 Great Britain Apr. 4, 1924

